1. Field of the Invention
The present invention relates to a honeycomb structure having a plug portion suitably used as a filter, especially to a honeycomb structure suitable for a filter removing particulate matter (hereinafter referred to as PM) emitted from a diesel engine (hereinafter the filter is referred to as DPF).
2. Description of the Related Art
The need to remove harmful substance in exhaust gas from an internal combustion engine, a boiler and the like is increasing in consideration of environmental problem. Especially, regulations for removing PM has become stricter in Europe, the United States and Japan. Thus use of a honeycomb structure as a DPF is broadening.
For the honeycomb structure used as such a filter, as shown in, for example FIGS. 6(a) to 6(c), it is known that a honeycomb structure 1 having porous partition walls 2 disposed so as to form cells 3 extending from one end face to the other end face, and a plug portion 4 disposed so as to plug the cell 3 at either end face. By such a configuration, fluid to be treated which flows into the cells from one end face 42 passes through the porous partition walls, and is discharged from the other end face 44 via other cells 3. At this time, the partition walls 2 trap PM functioning as filters.
When the above-mentioned honeycomb structure is used as a DPF, substance such as PM is heavily deposited on the end face of the cell to reduce an inflow area at an opening of the cell or to clog the opening. As a result, a pressure loss increases to cause deterioration of power and fuel efficiency of a diesel engine.
In particular, diesel engine emissions contain soluble organic fraction (hereinafter referred to as SOF) that is viscous substance. As the SOF adheres to the end face having openings, followed by the adhesion and deposition of other components on the SOF, the openings are reduced or clogged in inflow area, and this results in an increased pressure loss.
In using a honeycomb structure with plug portions as a DPF, the following plugged honeycomb structure is proposed. That is, the honeycomb structure, wherein the plug portion has a protruding portion protruding from the end face such that the portion tapers off toward the upstream side in order to prevent increased pressure loss due to the deposition of PM (see, e.g., JP-A-63-24731 and US2003/0093982). Although such a structure is effective in suppressing the deposition of non-viscous substances on the end face, the honeycomb structure having the plug portion as described above fails to sufficiently suppress the deposition of substances on the end face if viscous substances are contained in the gas.
Another structure proposed has partition walls deformed and bent so that the partition walls are joined together at the end of square cell passages to block the outlets of the cell passages (see, e.g., JP-B-3012167). Although such a structure is effective in suppressing the clogging, the deformed and bent portions are difficult to form. Further, a structure that could offer a further enhanced strength in the deformed and bent portions is desired.